Families of willow ptarmigan join to form flocks in September. The ptarmigan then begin to move around more than in the nesting season. Females and males tend to separate in late September and October; the females, usually in small groups, seek food and shelter at lower elevations. In most parts of Alaska these movements to and from summer ranges encompass only a few miles. Hens that nest or were reared on the north slope of the Brooks Range move up to 100 miles southward in late fall, wintering on the south side of the Brooks Range in the low hills and wooded valleys north of the Yukon River in the east, or in the valleys of the Noatak and Kobuk Rivers to the west. Males of these same populations also largely abandon summer ranges, but do not go as far south as the females. The south-tending migrations take place in October and November. The northward movements begin in February, reach a peak in April, and are finished by mid-May.